Mr. Ee, 30, worked at an Internet company in SoHo, and had been competing with a colleague to see who could lose more weight. But they had both stopped going to the gym because of long hours at the office. In search of more motivation, they turned up the pressure: they entered into a formal one-month wager to see who could cut the higher percentage of their body mass index.

“It got pretty competitive,” said Mr. Ee, who weighed 248 at the time. “We used to do push-ups every hour in front of each other’s cubicles. And he used to leave really good chocolate on my desk.”

His co-worker, Daniel Fries, won the first month, after losing about 16 pounds to Mr. Ee’s 10, and Mr. Ee paid him $20.

Then, Mr. Ee said, “it got serious.”

Six co-workers joined them in another weight-loss competition. “I told them this was going to be dirty,” Mr. Ee said. There were weekly weigh-ins by an outside record keeper. Mr. Ee finished second in that bet, after losing an additional four pounds. Along the way, Mr. Ee’s officemates enjoyed taunting one another with comments like, “Are you sure you want to eat that bagel?” and “Why don’t you get a cheeseburger for lunch and I’ll get a salad?”

It was an eight-player example of a diet bet, in which those seeking to lose pounds give themselves a new incentive: money. If they don’t lose more weight than the competition, they lose cash. Internet sites that facilitate diet betting have seen an increase in traffic, and recent studies have supported what Mr. Ee and his co-workers discovered: diet bets work for many people who couldn’t seem to shed pounds any other way.

“Diet betting is definitely becoming more popular among friends, relatives and co-workers,” said Joy Bauer, author of “Joy’s Life Diet: Four Steps to Thin Forever,” and founder of Joy Bauer Nutrition, a firm with offices in Manhattan and Westchester County. “It makes life easier if everyone around you is cutting calories, and the amicable competition keeps people driven. You are less likely to eat bad things from the candy jar.”

Most diet bettors agreed that while losing weight was the ultimate goal, winning the bet — and pocketing the winnings — soon became the main reason they stuck to their diets.

“I wanted to win, and I blew everyone away,” said Christopher Fallon, 36, a medical sales representative from West Orange, N.J. Mr. Fallon participated in a three-month diet bet with nine other colleagues, everyone contributing $100 to a winner-take-all pool. At a sales meeting a few weeks before the end of the bet, Mr. Fallon’s fellow bettors realized that he was way ahead.

“When I saw Chris at the gym at 6 a.m. looking skeletal, I knew it was over for me,” said one colleague, Carolyn Kramaritsch.

Mr. Fallon admitted that he enjoyed vanquishing his peers even more than losing the pounds. “I didn’t even need to lose much weight,” he said, “but when I saw everyone else, I thought, ‘I just won $900!’ ”

A study in the December issue of the Journal of the American Medical Association found that people who had financial incentives to lose weight were much more successful at dieting than those who did not.

The study created two groups of dieters with financial incentives, as well as a control group. Dr. Kevin Volpp, co-author of the study and an associate professor in behavioral economics at the University of Pennsylvania School of Medicine and the Wharton School, said the prospect of losing or gaining money was a significant incentive for dieters, especially when the money was not rewarded until the end of the period.

Weekly feedback was also an important component of the study, Dr. Volpp said, as was the very simple fact that people do not like to lose money.

Similarly, two Yale professors, Dean Karlan and Ian Ayres, studied the effects of commitment contracts on quitting smoking and then applied that research to dieting, developing a business around their theories. They teamed up with a graduate student, Jordan Goldberg, 24, and started StickK.com last year, a Web site that motivates people to make changes in their lives by signing contracts: if they fail in their goals, it costs them money.

The site has more than 23,000 users, the highest percentage of whom (42 percent) have commitment contracts for losing weight. The site takes credit card information up front and charges your card weekly if you fail to meet your goal. You can designate someone to be your referee — a friend, co-worker or spouse, for example — but in the end, if they fail to do their jobs, then StickK.com takes your word for it.

Bettors can also designate a friend or charity as recipients of their lost money, but they can also choose a “foe or anti-charity.” As the Web site states, “Wouldn’t it just kill you to hand over your hard-earned money to someone you can’t stand?”

You can choose from a list, which includes both the George W. Bush and Bill Clinton presidential libraries. The anti-charities have the highest success rate of all the StickK.com bets (85 percent), said Mr. Goldberg, chief executive of the company, adding, “Some people respond better to negative reinforcement.”

Pablo Juarez, 31, and Paul Sanborn, 40, co-workers at a software company in Washington, made separate diet contracts with StickK.com, designating each other to receive any money they lost. “We decided to bet $1,000 for 40 weeks, at a pound a week,” Mr. Sanborn said. “What drove us to be successful was the money and the social pressure. We have a scale in my office, and our office friends keep track of it.”

Mr. Juarez said: “I got caught up in the competition. I was cocky at first, but then I missed two weeks.” Both men lost more than 25 pounds.

Mr. Sanborn says he is particularly motivated to be successful so he can get money from Mr. Juarez that he is already owed. “When we were on a trip to Spain,” Mr. Sanborn said, “I got stuck with a 44-euro bill one night because Pablo was on the dance floor. Now when I’m on the StairMaster, I think, ‘I’m going to get those 44 euros back.’ ”

TWO other new Web sites, fatbet.net and makemoneylosingweight.com, are designed for dieters who want to challenge one another. The sites, which are free, have formalized the group-diet betting process by providing a forum for publicly tracking weight and setting specific incentives, although neither site handles the monetary part of the bet.

“Betting makes the approach to weight loss more acceptable for men,” said Prof. Geoffrey Greif of the University of Maryland School of Social Work and author of “Buddy System: Understanding Male Friendships.” “These guys are repeating what they did as kids on the playground to see who can run faster or throw the ball harder. And it’s also a safe way for men to be close.”

That doesn’t mean women don’t do it. Carol Lewis, 45, who works part time for the Federal Reserve in Boston, made a fat bet last summer with her husband. “He sent an e-mail challenge to me: 12 pounds for him and 6 for me,” Ms. Lewis said. They formalized their bet on fatbet.net and let the games begin.

The deal was that if Ms. Lewis lost, she had to wear a shirt to their sons’ soccer games that said: “I lost a fat bet to my husband. I am a big loser,” but she had not thought of a wager for him. They both lost some weight, although her husband lost more. Mr. Lewis is now involved in a new bet with other people.

“We razzed each other a lot,” Ms. Lewis said. “I baked a few cakes to try to tempt him, but he wouldn’t touch them so I ate them myself. But sometimes when I went to get a cookie, he’d look at me, and I’d put it down and say, ‘I’m not going to lose this bet!’ ”